With respect to mechanical parts used for automobiles, construction machines, and other various industrial machines, parts that are required to have particularly high strength are conventionally subjected to a surface-hardening heat treatment (case hardening treatment) such as carburizing, carbonitriding, or nitriding. For these applications, usually, case hardened steels specified by JIS, such as SCr, SCM, and SNCM, are used. The steel is formed into a desired part shape by machining such as cutting or forging and then subjected to a surface-hardening heat treatment as mentioned above, followed by a finishing process such as polishing, whereby a part is produced.
In recent years, in such mechanical parts, it has been desired to reduce the production cost, shorten the lead time, and reduce the CO2 emission during production, for example. Accordingly, part-forming methods are being changed from conventional cutting or hot forging to cold forging, and excellent cold forgeability is thus required. In addition, in a case hardened steel specified by JIS, the coarsening of crystal grains occurs due to the surface-hardening heat treatment after cold forging. Thus, it is also important to suppress the coarsening of crystal grains. In order to solve the problem of the coarsening of crystal grains, there is a conventionally used technique in which elements such as Al, Nb, and Ti are added to finely disperse precipitates such as AlN, Nb (CN), and TiC, and such fine precipitates are used to stop the migration of the crystal grain boundary (e.g., Patent Documents 1 to 8).
Japanese Patent Application Laid-Open Publication Nos. 2007-217761, 2006-307271, 2006-307270, 2007-321211, 2004-183064, 11-335777, 2006-161142, and 2007-162128 each disclose that the coarsening of crystal grains can be prevented by controlling the number of Nb- and/or Ti-containing precipitates having a predetermined grain size or composition (carbides, carbon nitrides, etc.) within a predetermined range. Although the disclosures have some preventive effects on the coarsening of crystal grains, cold forgeability has not yet been sufficient.